Filtering Options

In order to avoid confusion, the following changes have been implemented to the Verilog code:
- renamed the "rom_*" ports and defines to "pmem_*" (program memory).
- renamed the "ram_*" ports and defines to "dmem_*" (data memory).

In addition, in order to prevent potential conflicts with the Verilog defines of other IPs, a Verilog undefine file has been created.